Instrument reference system

ABSTRACT

A method and system for calibrating a motor-driven instrument pointer ( 8 ) against a stop ( 5 ) without using feedback includes, in a first power state, driving the pointer ( 8 ) at full torque against the stop ( 5 ), then driving the pointer ( 8 ) against the stop at a reduced torque for a predetermined radial distance so at the pointer ( 8 ) is caused to be held against the stop ( 5 ). A third driving step, at reduced torque for a further predetermined radial distance, may be used in a second power state to move the pointer ( 8 ) against the stop ( 5 ).

TECHNICAL FIELD OF THE INVENTION

This invention relates to an instrument reference system and moreparticularly relates to a system for calibrating an instrument pointerdriven by a motor against a mechanical stop without using feedbackcontrol. The invention will be described with reference to an automotiveinstrument by way of example only but it will be understood that theinvention is not limited thereto.

BACKGROUND OF THE INVENTION

Calibration of a motor, such as a stepper motor, used in an instrumentor equipment of the type used for automotive instrumentation, isimportant for correct operation of the instrument. Typically, anelectronic-controlled instrumentation may take the form of a mechanicaldial comprising a pointer driven by a stepper motor. To ensure properfunction, the pointer location must be predictable. As is known fromexisting systems, an inexpensive way of achieving this, without usingany form of feedback, is to incorporate a mechanical stop as part of themechanism and to locate the pointer relative to this stop at “power on”.

One known system for calibration of a stepper motor in automotiveinstrumentation provides that, at initial power up, the motor is drivenat least a few degrees against the stop to ensure that it has madecontact with the stop. To avoid jitter (small vibrations of the motorshaft) at the stop the frequency of the rotating magnetic field isincreased quickly beyond that at which the motor can synchronise itsmovement with the field.

The drawback of this system, however, is that the jitter is most evidentif the motor is already against the stop when the referencing procedurecommences. Another drawback with this technique is that it relies on thepointer remaining against its mechanical stop once it has reached it. Inpractice it has been found that the pointer has a tendency to ‘bounce’up to several degrees off the stop, especially when the motor has beenmoving at high speed when it hits the stop. Once this has occurred, themotor can not re-synchronise with the high frequency rotating field andremains stationary away from the reference position.

One possible solution to the ‘bouncing’ problem is to drive the motorand hence the pointer at a sufficiently low speed. This, however, is notdesirable as it can take considerable time where the field must rotatefor sufficient time to guarantee that the motor has reached the stop. Inautomotive instrumentation, this delay is particularly unsuitable.

SUMMARY OF THE INVENTION

It is therefore desirable to provide an improved calibration system forinstrumentation, without using feedback control.

It is further desirable to provide a system with reduced jitter of thedriving motor.

It is also desirable to provide a system which calibrates quickly andwhich can be produced at an economical cost.

According to one aspect of the invention there is provided a method ofcalibrating an electronically controlled, electric motor driveninstrument comprising the steps of:

a. connecting electrical power to the instrument,

b. driving the motor at full torque for a first predetermined radialdistance whereby the motor or a pointer driven thereby contacts a stop,

c. driving the motor at a reduced torque for a second predeterminedradial distance whereby the motor or pointer is caused to be heldagainst the stop.

Preferably, the first predetermined radial distance is between 180° and360°, more preferably between 240° and 350°.

Preferably, the second predetermined radial distance is between 1° and90°, more preferably about 10°.

In one form of the invention, a two stage calibration is providedwhereby the instrument is calibrated on initial connection to anelectrical power source as described above, and then undergoes a secondcalibration when electrical circuits with which the instrument isassociated are switched on. With this arrangement, the secondcalibration comprises the steps of:

d. connecting the instrument to at least one electrical circuit forwhich the instrument is to be associated,

e. driving the motor at a reduced torque for a third predeterminedradial distance whereby the motor or pointer contacts the stop, and

f. driving the motor at a reduced torque for said second predeterminedradial distance to cause the motor or pointer to be held against thestop.

Preferably, the third predetermined radial distance is between 10° and120°, more preferably about 90°.

According to another aspect of the invention there is provided a systemfor calibrating an instrument or equipment operated by an electricmotor, such as a stepper motor, comprising:

a pointer means engageable with said electric motor and adapted to bedriven thereby,

a stop means positioned at a desired reference point wherein,

in a first powered state, the motor is accelerated at a maximum torquefor a first predetermined radial distance to move said pointer towards aposition whereat said stop is engaged,

thereafter, said motor is re-accelerated at a reduced torque for asecond predetermined radial distance whereby the pointer is caused to bemoved and held against said stop.

In one form of the invention, a two stage calibration is providedwhereby, after an initial calibration, which occurs as soon as theinstrument is first connected to a power source, it then undergoes asecond calibration when electrical circuits with which the instrument isassociated are switched on. In accordance with this aspect of theinvention, the system includes a second calibration comprising,

in a second powered state in which the instrument is connected to atleast one electrical circuit for which the instrument is to beassociated, means for accelerating the motor at a reduced torque for athird predetermined radial distance whereby the motor or pointer driventhereby contacts the stop, and thereafter driving the motor at a reducedtorque for said second predetermined radial distance to cause the motoror pointer to be held against the stop.

Preferably, the third predetermined radial distance is between 10° and120°, more preferably about 90°.

By calibrating the motor in this way, the need for a feedback control isavoided and calibration is achieved inexpensively, quickly and withoutjitter in both powered states.

In a preferred form of the invention, the electric motor is a steppermotor as used in an automotive instrumentation, where for correctoperation of the instrument, calibration of the stepper motor isrequired.

The first powered state is preferably when an automotive instrument isfirst connected to a power supply, such as a battery. The second poweredstate is preferably when an ignition circuit of a vehicle is switchedon.

The calibration is preferably executed in the first powered statewhenever a battery power connection is interrupted and thenre-connected. The calibration is preferably executed in the secondpowered state every time the ignition is switched on.

Further, in the first powered state, at maximum torque, a maximum motorrotation speed maybe of the order of 440°/s and said first predeterminedradial distance may be 350°.

At the reduced torque, the motor rotation speed is preferably up to amaximum of 165°/s and said second predetermined radial distance may be10°.

In the second powered state, at the said reduced torque, the motorrotation speed is preferably up to a maximum of 165°/s and said thirdpredetermined radial distance maybe 90°.

In the first and second powered states, the motor or the pointerpreferably contacts the stop when the motor is first rotated andcompleted at the radial distance of up to 350° or 90°. The pointer maybounce off the stop upon completion of the rotation.

In practice, the pointer may be driven by an intermediate gear,contained within the motor housing which, in turn, is driven by a rotorof the motor. The pointer preferably contacts the stop and remainscontacted or stationary against the stop once the motor has completedits second rotation at reduced torque, for 10° and/or when the referenceposition is reached.

The pointer means may further comprise a pointer gear and the stop maycomprise a mechanical stop. The stop may be located within the motorhousing and contacted by the rotor of the motor, or by a co-operatingpart on the motor shaft, output gear or even an intermediate gear. Forsimplicity of description, however, reference will be made herein to thepointer contacting a stop, but it will be understood that this referenceis not limiting.

The processing means preferably comprises a microprocessor whichprocesses and controls the motor operation during calibration and normaloperation.

It will be understood that during calibration of the stepper motor, therotation of the rotor of the motor is in the opposite direction tonormal instrument operation.

In order that the invention is more readily understood, a preferredembodiment will now be described with reference to the drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic drawing of an instrument with a stepper motorembodying the invention,

FIG. 2 is a process flow chart illustrating the calibration process ofthe stepper motor of FIG. 1 in accordance with one embodiment of theinvention, and

FIG. 3 is an illustrated sequence of the process flow of FIG. 2.

DESCRIPTION OF THE PREFERRED EMBODIMENT

The invention will be described with reference to an automotiveinstrumentation stepper motor driving a pointer that rotates about anaxis. The instrumentation may be, for example, a speedometer of avehicle, a Rev counter, a fuel gauge, or any other instrument or gauge.

The general layout of the instrument system is shown in FIG. 1. Anelectronically controlled instrumentation 7 comprises a stepper motorhousing 1 having a rotor 2 and an intermediate gear 3. A pointer 8 withintegral gear 4 is engaged with the intermediate gear 3 and is adaptedto rotate about the axis of the integral gear 4. A mechanical stop 5 ispositioned at a desired reference or zero point, and a microprocessor 6provides control signals to the motor. While the stop 5 is shown asbeing contacted directly by the pointer 8, it will be understood thatthe stop may be provided in the motor housing 1 and contacted by themotor rotor 2 or a co-operating part driven thereby or by theintermediate gears.

FIG. 2 shows the process flow chart by which the microprocessor 6controls the calibration in accordance with the state of the power tothe system and the position of an ignition switch which controls theelectrical systems for which the instrument 7 is to be used.

FIG. 3 illustrates the various calibration stages in accordance with the“power on” and “ignition on” parameters. These are as follows:

1A—Power connected first time—Start position

1B—First stage acceleration towards Stop 5.

1C—Bounce off Stop 5.

1D—Second stage acceleration

1E—Pointer positioned against Stop 5.

2A—Ignition on—Start position.

2B—Third stage acceleration.

2C—Bounce off Stop 5—leads to 1D.

Referring to FIGS. 2 and 3, an embodiment of the invention incorporatesa microprocessor 6 which is programmed to calibrate the instrumentationwhen power is first connected to the instrument 7. In practice, thisoccurs when the battery is first connected in a vehicle or when it isreplaced or when it is disconnected and re-connected. Effectively, it isthe state where initial power is provided to the system. In the no-powerstate, there is no power to the instrument 7, the microprocessor 6 isoff and the motor 1 has no current.

Once power is connected to the instrument 7, the microprocessor 6assumes a low power standby mode and completes a logic check thatdetermines that the power has been re-connected after a powerinterruption. If the logic determines a new power connection, themicroprocessor 6 commences the calibration process in accordance withthe logic program to commence a first stage calibration.

The microprocessor contains a pre-programmed series of commands formingan algorithm which is executable when initial conditions are satisfied.The process of programming the microprocessor will not be described asthis will be known to a person skilled in this art.

In the first stage calibration, at commencement, it is assumed that thepointer 8 could be anywhere within the limits of its rotation. As shownin FIG. 3, in the start position 1A at “power on”, the pointer issubstantially away from the mechanical stop 5. (For simplicity, themicroprocessor 6, instrument dial 7 and motor housing 1 are not shown inthis drawing.)

The first stage calibration, shown at stage 1B, comprises acceleratingthe motor (rotor 2) at maximum torque and up to a maximum speed of440°/s for a total distance of about 350°. The motor rotation is in thenegative direction as compared to instrument operation, and it isassumed that the pointer 8 will probably bounce off the stop 5, as shownin position 1C in FIG. 3. To fully reference or zero the pointer 4 asecond stage calibration 1D is executed. This time, the motor 2 isaccelerated at a reduced torque and up to a maximum speed of about165°/s for a total distance of about 10° in the negative direction. Thisacceleration results in a smaller referencing or zeroing movement atreduced torque to move the pointer against the stop 5 and to eliminatefurther bouncing off the stop 5. This movement also reduces jitter atthe stop 5.

Once the second stage is complete, it is assumed that the pointer 8 isagainst its mechanical stop 5. Testing of this system has shown that thepointer, after the second stage, is referenced at the stop. The motorcan now be operated in a normal state during which it can respond toinput. Generally the motor will be operated with maximum torque.

In a second aspect of this embodiment, referencing of the instrument isrequired each time the vehicle ignition circuits are energized to ensurethat the instrument pointer 8 is engaged against the stop 5, as it ispossible for the pointer to move from the reference position duringperiods when the ignition circuits are “off”. Typically, the ignitioncircuits are energized when a driver of a vehicle turns the ignition keyto an “on” position. Generally, the microprocessor 6 and the instrumentmotor will be powered all the time the battery is connected, but, unlessthe battery circuit is interrupted by being disconnected, or a fuse isreplaced, etc., no referencing program will be initiated. Hence, asecond stage procedure is required to ensure the instrument is correctlycalibrated whenever the ignition circuits are activated.

For the second stage calibration, it is assumed that the motor isgenerally less than 90° away from its zero point. As shown is FIG. 3, atposition 2A, at the start position for “ignition on”, the pointer 8 isless than 90° from stop 5. In practice, the pointer may have movedslightly off the stop due to a force being applied, such as a vibration,shaking etc. of the instrumentation while the ignition circuits areswitched off. When the ignition is switched on, the microprocessor 6logic verifies that the ignition is on, as indicated in FIG. 2.

The instrument motor is then accelerated at a reduced torque and up to amaximum speed of 160°/s for a total distance of 90° in the negativedirection, as shown at position 2B in FIG. 3. Again, after this firstacceleration, it is assumed that the pointer 8 has reached the stop 5and has probably bounced off it 2C in FIG. 3. To fully reference or zerothe motor, the second stage shown at position 1D is executed, this beingthe same second stage of the previous, “power on” procedure, where themotor is accelerated at reduced torque up to a maximum speed of 160°/sfor a radial distance of 10°.

The embodiment described provides a system for calibration with reducedor no jitter in the motor, by using a two stage process. Further,calibration can be achieved quickly and inexpensively, as no feedbackcontrol is used and varying pointer dynamics can be reliably handledaccommodating different degrees of the pointer 8 bouncing off the stop5.

The invention has been described with reference to an instrument drivenby a single stepper motor. It is to be understood, however, that aplurality of stepper motors can be simultaneously controlled andcalibrated.

Further, since modifications within the spirit and scope of theinvention may be readily effected by persons skilled in the art, it isto be understood that the invention is not limited to the particularembodiment described, by way of example, hereinabove, and anymodifications which come within the scope of the invention shall bedeemed to be within the ambit of the above description.

What is claimed is:
 1. A method of calibrating an electronicallycontrolled, electric motor driven instrument, comprising the steps of:(a) connecting electrical power to the instrument, (b) driving the motorat full torque for a first predetermined radial distance whereby themotor or a pointer driven thereby contacts a stop, (c) driving the motorat a reduced torque for a second predetermined radial distance wherebythe motor or pointer is caused to be held against the stop.
 2. A methodaccording to claim 1, wherein the first predetermined radial distance isbetween 180° and 360°.
 3. A method according to claim 2, wherein thefirst predetermined radial distance is between 240° and 350°.
 4. Amethod according to any one of the preceding claims, wherein the secondpredetermined radial distance is between 1° and 90°.
 5. A methodaccording to claim 4, wherein the second predetermined radial distanceis 10°.
 6. A method according to claim 1, further comprising the stepsof: (d) connecting the instrument to at least one electrical circuit forwhich the instrument is to be associated, (e) driving the motor at areduced torque for a third predetermined radial distance whereby themotor or pointer contacts the stop, and (f) driving the motor at areduced torque for said second predetermined radial distance to causethe motor or pointer to be held against the stop.
 7. A method accordingto claim 6, wherein the third predetermined radial distance is between10° and 120°.
 8. A method according to claim 7, wherein the thirdpredetermined radial distance is 90°.
 9. A system for calibrating aninstrument or equipment operated by an electric motor, such as a steppermotor, comprising: a pointer means engageable with said electric motorand adapted to be driven thereby, a stop means positioned at a desiredreference point wherein, in a first powered state, the motor isaccelerated at a maximum torque for a first predetermined radialdistance to move said pointer towards a position whereat said stop isengaged, thereafter, said motor is re-accelerated at a reduced torquefor a second predetermined radial distance whereby the pointer is causedto be moved and held against said stop.
 10. A system according to claim9, wherein in the first powered state, at maximum torque, a maximummotor rotation speed is 440°/s, and said first predetermined radialdistance is 350°.
 11. A system according to either of claims 9 or 10,wherein at the reduced torque, the motor rotation speed is up to amaximum of 165°/s and said second predetermined distance is 10°.
 12. Asystem according to claim 9, further comprising: in a second poweredstate in which the instrument is connected to at least one electricalcircuit for which the instrument is to be associated, means foraccelerating the motor at a reduced torque for a third predeterminedradial distance whereby the motor or pointer driven thereby contacts thestop, and thereafter driving the motor at a reduced torque for saidsecond predetermined radial distance to cause the motor or pointer to beheld against the stop.
 13. A system according to claim 12, wherein thethird predetermined radial distance is between 10° and 120°.
 14. Asystem according to claim 13, wherein the third predetermined radialdistance is 90°.
 15. A system according to claim 12, wherein, in thesecond powered state, at the said reduced torque, the motor rotationspeed is up to a maximum of 165°/s and said third predetermined radialdistance is 90°.
 16. A system according to claim 9, wherein, in thefirst and second powered states, the motor or the pointer contacts thestep when the motor is first rotated and completed at the radialdistance of up to 350° or 90°.
 17. A system according to claim 9,wherein the first powered state is when an automotive instrument isfirst connected to a power supply, such as a battery.
 18. A systemaccording to claim 12, wherein the second powered state is when anignition circuit of a vehicle is switched on.
 19. A system according toclaim 9, wherein the calibration is executed in the first powered statewhenever the battery power connection is interrupted and thenre-connected.
 20. A system according to claim 12, wherein thecalibration is executed in the second powered state every time anignition circuit of a vehicle is switched on.